Photovoltaic (PV) modules, also known as solar panels, are used in solar power installations for converting sunlight to electricity. Such installations range from small rooftop systems on residential or commercial buildings to large utility-scale facilities including hundreds of thousands or millions of PV modules. Collectively, we refer to these as “solar power plants.”
Frequently solar power plants employ performance monitoring systems to monitor and compare output power to expectations, allowing fault conditions or underperforming equipment to be identified and repaired, especially in large commercial or utility-scale facilities. Since the instantaneous electrical output of solar power plants is related to the incident solar irradiance, such performance monitoring systems include reference devices to measure the incident solar irradiance. Large solar power plants may include many reference devices in order to monitor irradiance in different sections of the plant as well as to provide measurement redundancy. Frequently reference device data on solar irradiance are integrated over time to yield solar insolation, the energy received per unit area over a given time period, also referred to as irradiation.
Several types of reference devices are in use for measurement of solar irradiance in a solar power plant.
One of the most widely used reference devices is the thermopile pyranometer. This device measures the temperature rise of an absorbing disk exposed to the incident solar irradiance. Pyranometers have a very uniform spectral response over the majority of the solar spectrum, and therefore provide a good measure of total incident solar radiation without regard to spectral variations. However, because of this and other factors, pyranometer measurements do not correlate perfectly with the output of a PV solar power plant. PV devices are sensitive to typical solar spectral variations. Such variations are caused by seasonal, geographic, and man-made atmospheric effects. The effect of typical solar spectral variations on the discrepancy between pyranometers and PV devices may be on the order of several percent. Furthermore, pyranometers and PV devices may show additional discrepancies due to differences in angular response, temperature coefficients, module soiling, and other parameters.
Another widely used reference device is the PV reference cell. This device functions by measuring the short-circuit current from a single PV cell encapsulated in a package, which typically contains a temperature sensor allowing the cell reading to be temperature corrected. Reference cells are typically calibrated in laboratory conditions where spectral response, linearity, and temperature coefficients may be precisely characterized. In contrast to pyranometers, which are spectrally insensitive and measure total radiation, reference cells are spectrally sensitive and measure the effective irradiance or usable irradiance available to the PV device. Reference cell readings therefore correlate very well with the electrical output of PV modules constructed from the same or similar PV technology, since PV modules are made from a group of PV cells in series and/or parallel combination.
However, PV reference cell devices are not widely available in all PV technologies for the purpose of matching to a particular PV module type. PV reference cells are typically fabricated using crystalline silicon PV devices. Their spectral response may vary from that of thin film PV devices such as those made from, e.g., cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS), or even from that of specialized crystalline silicon PV devices fabricated using different methods. Large differences in spectral response between the reference cell and the PV modules to be monitored can be reduced by adding a filter to the reference cell that adjusts its spectral response to more closely match that of a particular PV module type.
Another approach is to use a designated PV reference module as the reference device. In this case, the reference module may be chosen to be identical in technology and construction to the power-producing modules used in the solar power plant, and its spectral response and temperature coefficients will be the same as those of the monitored modules. The reference module is calibrated to determine its output as a function of incident solar irradiance. Like a reference cell, a reference module measures the usable solar irradiance received by the PV device, rather than the total irradiance. Use of a reference module has several potential advantages: the reference module may be selected directly from the same manufacturing line used to produce the monitored modules; the spectral and angular responses are exactly the same as the monitored modules; and the reference modules are constructed to withstand outdoor conditions for many years yet are typically available at lower cost than reference cells due to high volume manufacturing.
However, several practical difficulties arise with the use of reference modules.
The typical measurement approach for a reference cell or reference module is to hold the device near short circuit and measure current through a shunt resistance. Under these conditions, reference modules may degrade faster than reference cells, since at short-circuit lower-performing cells within a module may be forced into reverse bias by higher-performing cells and therefore dissipate power and generate heat, leading to degradation and/or eventual failure.
Furthermore, the linearity of reference module output with respect to light intensity may be inferior to that of a reference cell due to parasitic shunt and series resistances.
Furthermore, laboratory-based calibration and periodic recalibrations are considerably more expensive for reference modules than for reference cells, due to the larger size of reference modules and associated greater costs of removing them from service, shipping them to and from laboratories, and replacing them into service, as well as the greater risk of stress or damage to reference modules during handling and shipping due to their larger size and more complicated construction.
In view of the above shortcomings of existing methods for using PV reference modules to monitor field-installed PV modules in a solar power plant, there is a need for an improved system to measure and calibrate PV reference modules.